There is an option on twitter called "Find People". If you follow that link you will have a couple of options. If you are looking for people you know I would advise you to choose "Find on other networks" and then select the type of email you have and log in from there. Your list of contacts will receive the message you write in there and will be able to add you to their twitter.
You can also send an invite just for the email addresses you want.
Hope this helps.